Preparation of lower tetraalkyl quaternary ammonium nitrate



United States Patent Ofiice 3&7933? Patented Feb. 26, 1963 3,ll79,437 PREPARATHON F LOWER 'I'ETRAALKYL QUA'I'ERNARY AMMGNIUM NITRATE Ray S. Long, Concord, Caiif., assigncr to The Dow Chemical Company, Midland, Mich a corporation of Delaware No Drawing. Filed Feb. 26, 1959, Ser. No. 795,865

7 Claims. (Cl. 26b567.6)

This invention relates to a process for the preparation of .a quaternary ammonium nitrate and more particularly to the preparation of a lower tetraalkyl quaternary ammonium nitrate.

Lower tetraalkyl quaternary ammonium nitrates have aroused considerable interest in the field of rocket propulsion. These compounds are used primarily as components of liquid rocket fuels, particularly in liquid monopropellants where they contribute to both the oxidizing component and the fuel component. The most common method for the preparation of these compounds consists of reacting a tetraalkyl quarternary ammonium halide with silver nitrate in an aqueous solution. In this process the silver nitrate reacts With the tetraalkyl quaternary ammonium forming tetraalkyl quaternary ammonium nitra-te and an insoluble silver halide which precipitates out and is separated by filtration. Since such a process consumes both silver and the halide, it is costly and thus economically impractical for a large scale operation.

It is, therefore, an object of this invention to provide an efiicient process for the preparation of a lower tetraalkyl quaternary ammonium nitrate. A further object is to provide a process which does not require expensive reactants and is adaptable to a large scale operation.

The above and other objects are attained according to the invention by dispersing in an aqueous solution of a lower tetraalkyl quaternary ammonium halide an alkyl sulfonic acid or an alkyl aryl sulfonic acid to react the acid with the quaternary ammonium halide to form a tetraalkyl quaternary ammonium salt of the sulfonic acid. The quarternary ammonium salt of the acid is insoluble in the aqueous solution and precipitates out. The precipitated salt is then intermixed with an aqueous nitric acid solution which reacts with the quaternary ammonium salt of the sulfonic acid to form a quaternary ammonium nitrate and sulfonic acid. The sulfonic acid obtained is insoluble in the aqueous solution, while the quaternary ammonium nitrate dissolves in the solution and is later recovered by known means.

While the sulfonic acid may be dispersed in the aqueous halide solution to carry out the reaction of the acid with the tetraalkyl quaternary ammonium halide, the employment of an organic solvent which is immiscible with water to dissolve the sulfonic acid prior to the reaction greatly facilitates the contacting of the acid with the quaternary ammonium halide and its reaction with the halide. Preferably the solvent must also be one in which the quaternary ammonium halide is relatively insoluble. The solubility of the quaternary ammonium salt may not materially affect the reactions. However, it will be present with the tetraalkyl quaternary ammonium sulfonic acid salt formed by the reaction with the sulfonic acid and will either be lost or special steps will have to be taken to recover it. Aliphatic hydrocarbons, such as naphtha, gasoline, kerosene and the like, are most often used.

When a solution of the sulfonic acid in an organic solution immiscible with water is used, it is intermixed with the aqueous solution of the tetraalkyl quaternary ammonium halide to react the acid with the quaternary ammonium halide. A heterogeneous mixture is obtained with the quaternary ammonium salt of the sulfonic acid formed in the reaction being in the non-aqueous phase. The non-aqueous phase containing the quaternary ammonium salt of the sulfonic acid is separated from the aqueous phase and then intermixed with an aqueous nitric acid solution which reacts with the salt to form a quaternary ammonium nitrate and sulfonic acid. The sulfonic acid obtained is in an organic phase and the resulting quaternary ammonium nitrate is in an aqueous phase of a heterogeneous mixture obtained by intermixing the nonaqueous phase from the first reaction with the nitric acid solution. The aqueous phase is separated from the organic phase and the tetraalkyl ammonium nitrate is recovered from the aqueous solution.

The term immiscible, as used here in reference to the organic solvent and water, means that the solvent and water are essentially insoluble in each other.

Lower tetraalkyl quaternary ammonium nitrates having a total of from 4 to 7 carbon atoms and a general formula:

where R R R and R represent an alkyl radical having from 1 to 4 carbon atoms may be thus prepared. Illustrative examples of these compounds, which are finding wide utility as liquid rocket fuels, are tetramethyl ammonium nitrate, trirnethylisopropyl ammonium nitrate, and diethyldimethyl ammonium nitrate.

In the preparation of the nitrate, a corresponding lower tetraalkyl quaternary ammonium halide is used. The halide has a total of from 4 to 7 carbon atoms and a general formula:

where R R R and R represent a lower alkyl radical of from 1 to 4 carbon atoms as described above and X represents a halogen selected from the group consisting of bromine and chlorine. A bromide salt is preferred. These halide reactants may be prepared by known suitable methods, such as by the reaction of a tertiary amine with a monoalkyl halide.

A sulfonic acid which is essentially insoluble in water is preferably used for the reaction with the quaternary ammonium halide solution. While the solubility of the acid in water is relatively immaterial to the reaction, the amount dissolved is not generally recovered so that it represents a loss. The size of the alkyl substituent on the sulfonic acid determines to a great exient the solubility of the compound. Generally, an alkyl sulfonic acid which has less than 12 carbon atoms in the alkyl substituents has an appreciable solubility in water making this compound less desirable. If the total substituent carbon atoms are more than 50, the compound will generally lack suflicient solubility in the organic solvents so that these compounds are seldom used, if a solution of the acid is to be used. Acids which have alkyl substituents having from 15 to 25 carbon atoms are generally most suited with the alkyl aryl sulfonic acids, such as alkyl naphthalene sulfonic acids, being preferred.

Illustrative examples of sulfonic acids which are suitable are dinonyl naphthalene sulfonic acid, dodecyl benzene sulfonic acid, and heptadecyl sulfonic acid.

When the sulfonic acid is dissolved in an organic solvent which will not form a solution with water, techniques and equipment used in liquid-liquid extraction may be used in intermixing and reacting the sulfonic acid solution with the tetraalkyl quaternary ammonium halide. A countercurrent multistage contact operation is generally preferred. Usually 3 to 6 stages are used.

salt containing from a For contacting of the quaternary ammonium salt of the sulfonic acid with nitric acid, techniques and equipment employed in liquid-liquid extraction may likewise be used. Generally 2 to 4 stages are employed. The snlfo-nic acid-tetraalkyl quaternary ammonium halide equipment may be operated in series with the equipment used in contacting the quaternary ammonium salt of the sulfonic acid with the nitric acid. Thus, the quaternary ammonium salt of the sulfonic acid leaving the last sulfonic acid contact stage enters the nitric acid treating section where it is contacted with the nitric acid in a countercurrent multistage operation. the nitric acid treatment which contains sulfonic acid may be recycled to be contacted with more incoming quaternary ammonium halide, while the product of the tetraalkyl quaternary ammonium nitrate obtained as an aqueous solution from the last nitric acid treatment stage may be recovered from the solution by conventional methods.

A convenient method for the recovery of the product is by vacuum evaporation and crystallization. The crystals obtained may be washed with a liquid, such as acetone, which is a solvent for most of the reaction products but not for the desired tetraalkyl quaternary ammonium nitrate and then dried.

While the concentrations of the tetraalkyl ammonium halide solution and the sulfonic acid used may be as high as saturation, a solution containing from 10 to 20 weight percent of each of the tetraalkyl quaternary ammonium halide and the sulionic acid is preferred. With solutions containing the preferred concentration substantially complete conversion of the alkyl quaternary ammonium halide and the sulfonic acid may be obtained readily in relatively few contact stages. At concentrations containing greater than 20 weight percent, more contact stages have to be used to obtain substantially complete conversion. Solutions having as little as 0.01 weight percent may be used, but the disadvantage of handling large quantities of liquids at such a low concentration makes it desirable to use solutions containing at least 5 weight percent and preferably at least weight percent. Generally, sufficient amount of the sulfonic acid solution is used to give a slight stoichiometric excess of the acid.

Generally, the tetraalkyl quaternary ammonium salt of the sultonic acid obtained in the non-aqueous phase is contacted directly with the nitric acid solution without dilution or further concentration. However, if it is desirable the sulfonic acid salt solution may be diluted by addition of more of the organic solvent or concentrated by distilling off part of the solvent. A solution of the fraction of a weight percent to about a 2 molar solution may be employed. The concentration of the aqueous nitric acid solution used may likewise be widely varied. A solution containing from about 6 to 32 percent is especially suitable. It is generally preferred to employ a stoichiometric amount of a nitric acid solution containing from to weight percent of the acid.

In contacting the tetraalkyl quaternary ammonium halide with the solution of sulfonic acid, generally room temperatures are used. A higher or lower temperature may be employed, if desired. Temperatures up to about 50 C. may be beneficial in increasing the reaction rate and, improving the separation between the two phases. Generally no substantial advantage is gained by using a temperature below room temperature, especially when the cost of cooling is taken into account. The same applies when the tetraalkyl quaternary ammonium salt of the sultonic acid is contacted with nitric acid. While other temperatures may be used and certain advantages may be obtained using other temperatures, room temperature is preferred and generally used.

The following example further illustrates the invention but is not to be construed as limiting it thereto.

The organic phase from .salt with an alkyl sulfonic Example About milliliters of a 20 percent by weight solution of dinonyl naphthalene sulionic acid in kerosene was contacted with about 100 milliliters of a 0.966 molar (17 weight percent) solution of trimethylisopropyl ammonium bromide in water. Upon intermixing of the two solutions a heterogeneous two phase mixture was obtained. The organic phase was a 0.55 molar (9.5 weight percent) solution of trimethylisopropyl ammonium salt of dinonyl naphthalene sulfonic acid in kerosene. The aqueous phase contained hydrogen bromide and the unconverted quaternary ammonium halide.

' Five volumes of the organic phase so obtained was then intermixed with one volume of a 22 weight percent nitric acid solution. Upon iutermixing the organic phase with the aqueous nitric acid solution, an aqueous phase and an organic phase were obtained. The aqueous phase was separated from the organic phase and upon analysis was found to be a 2.9 molar trimethylisopropyl ammo: nium nitrate (51 weight percent) solution. This solution still contained approximately 7 percent nitric acid and could, of course, be used in succeeding stages to completely utilize the nitric acid. The organic phase obtained contained dinonyl naphthalene sulfonic acid which was recycled to be contacted with more of the tetraalkyl ammonium bromide. The aqueous solution containing the tetramethylisopropyl ammonium nitrate product was evaporated under vacuum until a thick slurry was obtained. The slurry thus obtained was filtered to recover the crystals which were then washed with acetone and dried in vacuum.

In the single stage contact of the trimethylisopropyl ammonium bromide with the aryl sulfonic acid and a single stage contact of the resulting sulfonic acid salt with nitric acid, 60 percent of the trimethylisopropyl ammonium bromide was recovered in the final product as the nitrate salt.

In a manner similar to that above other tetraalkylquaternary ammonium nitrates having up to a total of 4 to 7 carbon atoms as described above may be prepared by reacting the corresponding chloride or bromide acid and then converting the quaternary ammonium salt of the sulfonic acid to the nitrate form by use of nitric acid. In addition to the dinonyl naphthalene sulfonic acid other sulfonic acids which are essentially insoluble in water but soluble in organic solvents, such as dodecyl benzene sulfonic acid and heptadecyl sulfonic acid, may be used in place of the dinonyl naphthalene sulfonic acid. Aliphatic hydrocarbon solvents other than kerosene may also be used as a solvent for the alkyl sultonic acid.

What is claimed is:

1. A process for the preparation of a lower tetraalkyl quaternary ammonium nitrate which comprises dispersing in an aqueous solution of a tetraalkyl ammonium halide having a total of 4 to 7 carbon atoms and having a general formula:

where R R R and R represent a lower alkyl radical having from 1 to 4 carbon atoms and X represents a halogen selected from the group consisting of chlorine and bromine with a sulfonic acid selected from the group consisting of alkyl sulfonic acids and a kyl aryl sulfonic acids to react the tetraalkyl quaternary ammonium halide with the acid to obtain a tetraalkyl quaternary ammonium salt of the sulfonic acid, separating the tetraalltyl quater-- nary ammonium sulfonic acid salt from the reacted mass, iuterm'xing the separated tetraalkyl ammonium sulfonic acid salt with an aqueous solution of nitric acid to react the sulfonic acid salt with the nitric acid to form the tetratetraalkyl quaternary ammonium nitrate from the nitric acid reacted mass.

2. A process for the preparation of a lower tetraalkyl quaternary ammonioum nitrate which comprises interrnixing an aqueous solution of a tetraalkyl ammonium halide having a total of 4 to 7 carbon attoms and having a general formula:

where R R R and R represent a lower alkyl radical having from 1 to 4 carbon atoms and X represents a halogen selected from the group consisting of chlorine and bromine with a sulfonic acid selected from the group consisting of alkyl sulfonic acids and alkyl aryl sulfonic acids essentially insoluble in water dissolved in an inert organic solvent immiscible with water to react the tetraalkyl quaternary ammonium halide with the acid to obtain a tetraalkyl quaternary ammonium salt of the sulfonic acid in a non-aqueous phase of a heterogeneous mixture obtained by intermixing the two solutions, separating the non-aqueous phase from the remaining constituents of the reacted mass, intermixing the non-aqueous phase with an aqueous solution of nitric acid to react the tetralkyl quaternary ammonium salt of sulfonic acid with the nitric acid to form the tetraalkyl quaternary ammonium nitrate in an aqueous phase and sulfonic acid in an organic phase, separating the aqueous phase from the organice phase, and recovering the tetraalkyl quaternary ammonium nitrate from the aqueous phase.

3. A process according to claim 4 wherein the alkyl substituent of the alkyl aryl sulfonic acid contains from 12 to carbon atoms.

4. A process according to claim 2 wherein the solution of the tetraalkyl quaternary ammonium halide contains from 10 to 20 weight percent of the tetralkyl quaternary ammonium halide and the sulfonic acid is an alkyl aryl sulfonic acid dissolved in an aliphatic hydrocarbon solvent in an amount of from 10 to 20 weight percent.

5. A process according to claim 3 wherein the tetraalkyl quaternary ammonium halide is a tetraalkyl quaternary ammonium bromide.

6. A process according to claim 5 wherein the tetraalkyl quaternary ammonium halide is trimethylisopropyl ammonium bromide and the alkyl aryl sulfonic acid is dinonyl naphthalene sulfonic acid.

7. A process according to claim 2 wherein the sulfonic acid is an alkyl sulfonic acid having an alkyl substitucnt of from 12 to 50 carbon atoms.

No references cited. 

1. A PROCESS FOR THE PREPARATION OF A LOWER TETRAALKYL QUATERNARY AMMONIUM NITRATE WHICH COMPRISES DISPERSING IN AN AQUEOUS SOLUTION OF A TETRAALKYL AMMONIUM HALIDE HAVING A TOTAL OF 4 TO 7 ACARBON ATOMS AND HAVING A GENERAL FORMULA: 